This invention generally relates to small signal detectors and more particularly relates to an operational amplifier small signal detector operating in an offset voltage cancellation loop such that signals smaller than potential offset voltages may be detected.
In many applications a signal to be detected is of sufficiently low level that it is below or of the same magnitude as noise levels of the signal detector. This is particularly true if the detector is an operational amplifier or voltage comparator, both of which may have input voltage or current offset errors as much as 5 millivolts or 100 nanoamperes. A desired signal may be a 4 millivolt signal and may be entirely lost in the input offset voltage such that it cannot be detected.
To resolve this problem, others have developed unique circuits or amplifier strings to detect a desired signal. One such circuit employs direct current (DC) feedback to automatically adjust the threshold of a voltage comparator to remove the effects of an offset DC voltage thereby permitting detection of very small alternating current (AC) signals. However, the large time constant needed by this circuit for stability also causes an undesirable delay in circuit turn-on and in threshold adjustment for changing conditions.
Another previously known circuit modulates the weak desired signal with a strong known signal, filters and detects the product, and indicates the level of the desired signal by nulling a meter. This circuit is effective for high frequency signals but introduces complexity and expense which may be inappropriate for some applications.
In the application where a tone is to be detected from an audio signal such as the audio output of a radio receiver, a reliable and inexpensive detector which may be exposed to thermal, mechanical. and electrical stresses is desired. The invention hereinatter described has been conceived and developed to overcome these obstacles and environments.